Currently used electronic devices for monitoring weft yarn breakage in air jet weaving looms comprise at least one optoelectronic detector disposed in such a manner as to sense that the yarn has been correctly inserted. More exacly, the detector in question, disposed in the profiled loom reed at the predetermined height, senses the presence of the weft yarn when the free front end of said yarn, during insertion into the shed, reaches its sensitive field and passes through it.
The practical embodiments of such devices used up to the present time operate in accordance with the following general principles:
(a) by reflection on the yarn: in this case, the optoelectronic detector is constituted by an emitter element and a photosensitive element which are disposed in such a manner as to create a sensitive zone along the yarn passage channel in the reed, the state of this zone being disturbed by the passage of said yarn;
(b) optical barrier principle: in this case, two opposing photoelement face the yarn passage channel in the reed, and the weft yarn interrupts the sensitive field when it crosses it.
Both these systems have certain drawbacks. In particular, because of the physical dimensions of the photoelements, the optical barrier system requires the reed to be cut at the zone into which it is inserted, this involving high operating costs because even for slightly different manufactured fabric heights and/or loom heights it is necessary to use different reeds. The reflection system does not suffer from the said drawback because the detector is outside the reed. It is however more delicate in operation and of low overall reliability, in that it utilises light reflection by the yarn and not all yarns reflect in the same manner. In fact some of them (in particular black, opaque and similar yarns) reflect very little, to such an extent that the disturbance produced is minimal and cannot be detected.